Promising worlds in the solar system to search for extraterrestrial life

Exploring new worlds: where and how to look for extraterrestrial life. Scientists searching for life in the depths of the Solar System. How unique is the Earth and the solar system as a whole is a controversial question.

The search for extraterrestrial life in the solar system
The search for extraterrestrial life in the solar system.

The search for life in our solar system

The Earth's biosphere contains all the known ingredients needed to sustain life (as we know it). But the recent discovery of possibly biogenic phosphine in the Venus clouds reminds us that at least some of these ingredients exist in other parts of the solar system.

There are at least four promising cosmic bodies, where a thorough search for extraterrestrial life may eventually yield results.

The search for extraterrestrial life on Mars

The Red Planet is one of the most similar worlds of the solar system to the Earth. A day on Mars lasts 24.5 hours, polar ice caps grow and shrink depending on the season. A significant mass of the planet's surfaces was once buried underwater.

The discovery of a lake under the southern polar ice cap of Mars and methane in the Martian atmosphere (which varies depending on the season and even time of day) makes Mars an exciting candidate for life.

Biological processes can produce methane, but the real source of this gas on Mars is unknown. Scientists believe that once the planet was a much more favorable environment, but today Mars has a fragile, dry atmosphere, almost entirely composed of carbon dioxide.

It provides very little protection from solar and cosmic rays. However, Mars may have managed to keep some liquid water reserves under the surface, in which case life on the planet may still exist.

Can life survive on Europa?

This satellite of Jupiter was discovered by Galileo Galilei in 1610, along with three other larger moons. Europe is slightly smaller than our moon and turns around the gas giant at a distance of 670 000 km every 84 hours.

Europe is compressed continuously and stretched by the gravitational fields of Jupiter and other Galilean satellites. This process is known as tidal bending. 

It is believed that this moon is a geologically active world, like the Earth, because strong tidal curves heat the interior of the satellite.

Europe's outer layers (presumably 100 km thick) consist of water - part as an ice crust 10-30 km thick, and function as a subsurface liquid ocean. Below are the rocks, and in the center, presumably, it is a small metal core.

Evidence of the existence of the subsurface ocean can include geysers breaking through cracks in the ice, a weak magnetic field, and chaotic topography on the surface, which could be deformed by ocean currents spinning under it. 

This ice shield isolates the underground ocean from the extreme cold and cosmic vacuum and the ferocious radiation belts of Jupiter.

At the bottom of this ocean world, we can find hydrothermal vents and volcanoes. On Earth, such objects often support vibrant and diverse ecosystems.

Enceladus

Like Europe, Enceladus is an ice-covered satellite with a liquid water sub-surface ocean. Enceladus revolves around Saturn and first attracted scientists' attention as a potentially inhabited world after the unexpected discovery of huge geysers near the South Pole.

These jets of water come out of large cracks on the surface and, given the weak gravitational field of Enceladus, are splashed into space. This is clear evidence of underground liquid water storage.

In these geysers were found not only water, but also many organic molecules and, most importantly, tiny particles of solid silicate particles, which can only be present if the ocean's subsurface water is in physical contact with the rocky bottom at a temperature of at least 90 ˚C.

This is compelling evidence of hydrothermal vents on the ocean floor, providing the chemical composition necessary for forming life and localized energy sources.

Life on Titan

Titan is the largest satellite of Saturn and the only satellite in the Solar System with a dense atmosphere. Studies conducted by the Huygens probe showed that rains of liquid methane and other organic substances periodically occur in Titan's atmosphere.

The atmosphere of Titan consists mainly of nitrogen, an essential chemical element used to build proteins in all known life forms. 

Radar observations have revealed the presence of rivers and lakes of liquid methane and ethane and possibly the presence of cryovolcanoes - volcanic-like formations that spew out liquid water rather than lava. This suggests that Titan, like Europe and Enceladus, has a stock of liquid water under the surface.

At such a vast distance from the Sun, Titan's surface temperature is -180 ˚C - too cold for liquid water.

However, the abundance of chemicals available in Titan has given rise to speculation that life forms with fundamentally different chemical composition from terrestrial organisms may exist there.